The present invention relates to a method and apparatus for processing optically encoded signals, and in particular to the recognition of an optically encoded binary word.
The need to compare a digitally encoded word with a target word arises in a variety of different contexts within the overall field of optical communication and signal processing systems. One important application for such techniques is in the recognition of addresses in the header of a frame carried on an optical communications network. To enable use of such a network at very high bit is rates, e.g. 100 Gbit/s, it is important that the recognition can be carried out at ultrafast speed, e.g. 400 ps for a 40-bit word. To this end, a number of different approaches have previously been proposed. These include ultrashort pulse spectral phase encoding, optical correlation using spread spectrum and time division encoding techniques, and all-optical logic techniques using, for example, a soliton-dragging gate or a non-linear optical loop mirror. In the case of techniques of this latter sort, using non-linear fibre devices, it has been possible to process ultrafast optical signals. However, such devices suffer a significant disadvantage, in that they have long latency times, that is a long delay between input and output signals.
One approach to codeword recognition is disclosed in PCT/CH91/00122. Whilst recognising the need to provide speed and sensitivity, this prior system relies upon a hybrid electrical/optical system in which the word to be recognized W is applied in the optical domain to a gate switched by an electrical control sequence corresponding to the target word T. The output from the gate is summed incoherently in a photodetector. In practice, the speed of recognition achievable with such a system is severely limited.